Validate CRC and check for corrupted data

[turicas]

Currently, there's no CRC validation on read_hint_from_data_file function. This verification adds the "atomicity" to the process and should be done. We should also catch exceptions on every struct.unpack.

Tasks:

• Validate CRC on read_hint_from_data_file function
• Check if CRC is being calculated correctly on pack_data
• Catch exceptions on every struct.unpack

[turicas]

I've made some tests in data entry creation on this gist, it may help with CRC checking.